N-terminal modification of certain proteins using transamination reactions has been reported. See for example, Dixon, N-Terminal Modification of Proteins A Review, Journal of Protein Chemistry, 1984, 3, 99-108, and references therein. Dixon reports that the transamination reaction occurs at a pH range of 5-7, is specific for N-terminal amines of α-amino groups and can be used for removal of N-terminal residues of proteins under non-denaturing conditions. See, Improved Conditions for the Removal of 2-oxacyl groups form the N-terminus of proteins, M. Sunde, M. J. Sparkes, and H. B. F. Dixon, Biochim. Biophys. Acta 1998, 1388, 45-52. Other reports of converting human growth hormones to the corresponding terminal keto amides have been reported. See O. Nishimura et al., An efficient chemical method for removing N-terminal extra methionine from recombinant methioylated human growth hormone, Chem. Commun. 1998, 1135-1136. Additional work by Gilmore et al., Angew. Chem. Int. Ed. 2006, 45, 5307-5311 and Christman et al. J. Mat. Chem. 2007, 17 2021-1627 using pyridoxal phosphate shows the oxidation of the N-terminal of select proteins.
However, these methods have a variety of drawbacks. For example, the methods employ harsh reaction conditions under which it is difficult to maintain the folded structure of proteins necessary for their activity. The methods lack the site-specificity and therefore it is difficult to control or limit the reaction to the N-terminal of the proteins. The methods are not generally applicable to a variety of proteins but only work with a selected number of proteins thereby severely limiting the scope of these methods.
Therefore, there remains a need for methods for modifications of proteins that are tolerable, site-specific and can be applied to a variety of proteins. There also exists a need to utilize the resulting adducts of such modified proteins for screening and/or therapy of disorders such as cancer, Crohn's disease, arthritis, atherothrombosis and plaque rupture.